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Turbulence triggers birth

of massive stars

DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 24 February, 2009

By probing the newborn stars in the constellation Serpens Cauda, scientists using the Submillimeter Array have captured the first moments of massive star formation.

While scientists know the basics of star formation - a cloud of cosmic gas draws itself together, growing hotter and denser until nuclear fusion ignites - how massive star formation is triggered and what determines how many stars form from a single cloud is still a grey area.

The Trapezium cluster is an example of a group of new stars lurking within the constellation Orion. Each star shines with the brilliance of 100,000 Suns and contains 15 to 30 times as much material as the Sun. Image: ESO.

New data from the Submillimeter Array (SMA) is helping astronomers begin to answer these questions. The Harvard Smithsonian Center for Astrophysics (CfA) astronomers studied two dusty regions of star formation in the constellation Serpens Cauda, located 15,000 light years away. One region revealed significant heating, indicating that massive new stars must have already formed. The other region had ample material to form massive stars, but showed little signs of star formation, meaning that it is at one of the earliest stages yet identified in the birth of stars.

"The SMA enables us to see the dust and gas in the cocoon with amazing details, and to probe the initial stages of massive star formation," says Smithsonian astronomer Qizhou Zhang, who is lead author on the report that has just been accepted for publication in The Astrophysical Journal.

Another piece of the picture of star formation is gravity, which acts to fragment the contracting cloud of gas and dust into smaller and smaller pieces, creating clusters of stars. Such fragmentation may also inhibit the formation of massive stars. As a result, some theorists propose that massive stars must form from collisions of smaller protostars.

The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution and the Academia Sinica.

There are two forces that counteract gravity to suppress the fragmentation of the cloud: thermal pressure from the heat of protostars, and turbulence, which may allow massive stars to form directly from accretion. Previous work suggested that thermal pressure was the stronger influence, but the new SMA study finds that turbulence is more important, at least at the spatial scales examined.

"What's unique about these SMA observations is that we can check some of the hypotheses for massive star formation against the observations for the first time," says Zhang. "Unlike what has been assumed in theoretical models, we found that fragmentation is suppressed in these clouds, not by stellar heating but rather by turbulence."

The team already has planned future studies. "We have just started to understand the initial conditions in distant, massive star-forming regions. A large survey that we have launched with the SMA will, in the near future, reveal the nature of more of such objects," said Thushara Pillai of CfA, a co-author of the report.

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